Electric lamps are commonly formed by enclosing the filament in a glass volume and sealing the envelope to the filament leads. The seal between the leads and the envelope is a persistent problem for lamp manufacturers. The envelope usually has a different coefficient of thermal expansion than that of the lead material. When the lamp is turned on, the envelope and lead material heat up, causing mechanical stress between the envelope and the lead. If the lamp is operated at moderate temperatures, a glass material may be used for the envelope, and glasses may be compositionally tuned to have agreeable thermal expansions. Lamps operated at moderate temperatures; however, do not produced high quality light, and are not generally electrically efficient.
If the lamp is designed to be operated at high temperature, the choices for envelope glasses is limited. Quartz may also be selected as a high temperature envelope material. Quartz has a low thermal expansion, so to seal with quartz, the lead needs a low thermal expansion. Molybdenum is the most common lead material, but molybdenum is expensive, so reducing the amount used is a cost advantage. A common lead seal structure uses a thin molybdenum foil positioned between the internal and external lead wires. The thin foil then seals well to the quartz. Foil seals are expensive to make, and because the foil is flexible, the positioning control of the filament and leads during assembly may be difficult. An alternative seal uses round molybdenum wire that seals to the quartz. The external facing end of the molybdenum wire is butt welded to a steel wire that extends from the envelope for electrical connection.
When a molybdenum and steel lead wire is used in a lamp seal, the steel lead end is not sealed to the envelope material. The steel wire may slip in the surrounding envelope material. Exterior mechanical forces on the steel wire can therefore be transmitted to the weld joint. Also, if the envelope and exposed steel lead are fixed to exterior structures, the expansion and contraction of the steel lead can also exert force on the butt weld. The butt weld can then be mechanically worked by pulling on the leads, for example, by moving the envelope with respect to the base, thereby, or by thermal cycling. Pulling on the leads may cause the butt weld to fail, or the envelope to lead seal to fail. A known solution to the butt weld failure is to formed dents on the steel lead. The dents mechanically lock the steel wire to the glass, or quartz. The dents then prevent the steel lead from slipping in the surrounding glass material, and therefore prevent the steel lead from transferring forces to the butt weld. Unfortunately, the dented steel lead wires are not always successful at preventing the transmission of forces to the butt weld, and lamps made with dented steel leads are known to still fail because of broken butt welds. There is then a need for a lamp seal structure using inexpensive materials, that is easily manufactured, while having a high probability of a long lasting seal.